The invention relates to a method and a device for producing a directed gas jet as they are known from Chun Hang Sin et al. xe2x80x9cSupercritical Fluid/Supersonic Jet Spectroscopy with a Sheath-Flow Nozzlexe2x80x9d, Analytical Chemistry, Vol 64, No. 2, Jan. 15, 1992 (1992-01-15), pages 233-238, XP000248258 ISSN: 0003-2700.
A fast on-line analysis for gaseuos samples is desirable in many areas of research but also in the industry. It could be used for research, for the surveillance of exhaust gases, waste combustion plants, roasting gases during roasting of coffee, headspace analysis of mineral oils and soil samples. The information received therefrom can be used as a parameter for the process control. Of particular interest are often the compounds with aromatic base structures such as polycyclical aromatic hydrocarbons (PAH) in exhaust gases of industrial combustion plants. Since different isomers of the various PAH have different environmental relevance or, respectively, toxicity, it is reasonable to detect them selectively.
For a rapid on-line analysis of gaseous samples, molecule-spectroscopic procedures using the supersonic molecule beam technique are particularly suitable. In this procedure, the sample gas beam is adiabatically expanded into a vacuum, which results in a reduction of the internal energy of the sample molecules. This reduction of the internal energy results in a reduction of the temperature, that is, the sample molecules are cooled by the adiabatic expansion. As a result, the energy bands become narrower and do not overlapxe2x80x94in contrast to samples, which have not been cooled. Since the energy required for the excitation of the molecules is different for different compounds and also for different isomers of a compound, the energy needed for the excitation of the molecules can be used for an isomer-selective identification. For example, by the excitation and subsequent photo-ionization (REMPI) by means of a narrow band laser, a very high optical selectively can be achieved in this way up to an isomer selectivity.
Generally, the supersonic molecule beam or jet is generated by the expansion of a continuous or a pulsed gas beam through a small nozzle into a vacuum. This method has been used so for mainly for spectroscopic examinations where the detection sensitivity does not play any role. Since, during expansion, the sample beam becomes rapidly wider, which results in a large reduction of the sample density, the achievable detection sensitivity is noticeably worse than with alternative inlet techniques such as an effusive gas inlet wherein the sample molecules are not cooled. The utilization of the selective supersonic molecule beam technique in the on-line analysis is therefore aimed at an improvement in the detection sensitivity.
A proposal herefor is offered in the article by S. W. Stiller and M. V. Johnston: xe2x80x9cSupersonic Jet Spectroscopy with a Capillary Gas Chromatographic Inletxe2x80x9d, Anal. Chem. 1987, 59, 567-572. Stiller and Johnston developed a coupling of gas chromatography (GC) and laser-induced fluorescence spectroscopy (LIF) with supersonic molecular beam techniques (JET). To this end, they use an arrangement, wherein a GC capillary extends into the center of a concentric guide tube for an auxiliary gas beam. The sample gas supplied by way of the capillary is added into the core (center axis) of the auxiliary gas beam. The auxiliary gas beam and the sample gas beam centered in the auxiliary gas beam focussed along the center axis thereof are continuously expanded into a vacuum through a nozzle with a narrowed tip, whereby a continuous supersonic molecule beam is formed.
The adiabatic expansion of the gas beam into the vacuum results in a cooling of the auxiliary gas and the sample gas molecules. As a result of the adiabatic cooling more sharply defined bands for the excited states of the molecules are generated. With a sharply defined energy (laser wavelength), then only certain sample molecules can be excited, which provides for a high optical selectivity. With the high optical selectivity, the sample molecules can partially be detected even in an isomer-selective manner.
The concentric narrowing down of the gas beam guide tube toward the tip opening into the vacuum chamber causes an additional focussing of the sample gas beam onto the center axis of the auxiliary gas beam so that the expansion into the vacuum results in a delayed spatial expansion of the sample gas beam. With the subsequent ionization or fluorescence excitation, a larger part of the sample molecules can be irradiated (higher sensitivity) without the need for a reduction of the effective cross-section for the excitation or, respectively, ionization by a spatial expansion of the laser beam (lower power density).
Although the sample gas density in the excitation or, respectively, ionization volume can be increased with the arrangement as described by Stiller and Johnston, the high-vacuum conditions are detrimentally affected by the continuous gas beam to such a degree that collisions between the sample molecules and, respectively, the auxiliary gas molecules make sensible measurements impossible. Furthermore, a large part of the sample gas, which passes between the laser pulses and which is not ionized and can therefore not be detected, is wasted.
Another apparatus for improving the detection sensitivity by employing the supersonic molecular beam technique is described by B. V. Pepich, J. B. Callis, J. D. Sheldon Danielson and M. Gouterman in the article: xe2x80x9cPulsed free jet expansion system for high-resolution fluorescence spectroscopy of capillary gas chromatographic effluentsxe2x80x9d, Rev. Sci. Instrum. 57(5), 1986, 878-887. Pepich et al. represent therein a GC-supersonic molecular beam coupling for the laser-induced fluorescence spectroscopy. As a result of the pulsed inlet, among others, a first increase of the sample volume employed for the analysis in comparison with the effusive inlet system is achieved. In order not to interrupt the GC flow by the pulsed inlet, Pepich proposes to introduce the sample into a pre-chamber in an effusive manner. Into this pre-chamber, a pulsed carrier gas is injected, which also provides for the gas flow needed for the expansion cooling. This carrier gas compresses the sample gas in the pre-chamber and pushes it, like a piston, through a small opening downwardly into an optical chamber where the fluorescence excitation takes place. As a result of the pulsed compression and injection of the sample gases into the optical chamber, a larger number of sample molecules can be reached by the subsequent laser excitation (increase of the detection sensitivity). The opening of the valves and the laser pulses must be synchronized in order for the compressed sample gas pulse to be excited by the laser pulse.
With the supersonic molecular beam technique an adiabatic cooling of the sample is achieved, whereby the selectivity of the method is substantially increased.
The setup selected by Pepich et al. facilitates also a repetitive, timely limited compressions of the sample in gas flow direction and improves the detection sensitivity also for this reason. However, it does not prevent the rapid spatial expansion of the sample gases which is typical for the supersonic molecular beam technique and as a result of which a large part of the sample gas is outside the ionization volume when excitation or ionization takes place. A widening of the laser beam is again not feasible because of the deterioration of the effective ionization cross-section.
It is the object of the present invention to provide a method and apparatus of the type described above wherein however a maximum particle density can be generated.
In a method for producing a directed gas jet wherein a guided sample gas beam is generated and an auxiliary gas beam is generated and directed and guided in the same direction as, but separated from, the sample gas beam, a pulsed carrier gas stream is generated and combined with the sample gas beam such that the sample gas beam is separated into spaced pulses which are embedded between the axially spaced pulses of the carrier gas beam and the carrier gas beam with the sample gas beam embedded therein is combined with the auxiliary gas beam such that the carrier and sample gas beam is radially enveloped by the auxiliary gas beam to from the directed gas jet of a carrier and sample gas pulses enveloped in the auxiliary gas beam.
The oriented guided auxiliary gas beam remains separate from the sample gas beam but extends in the same direction and both beams are combined over a certain distance. The lengths of the auxiliary gas and the sample gas beam guidance and the length for their combination needs to be adapted to the respective requirements. With an extended distance for the combination of the two gas beams (several centimeter), the sample gas and auxiliary gas are mixed to a greater degree than with a shorter distance (several millimeter). Depending on whether a mixing is desired or such mixing is to be avoided, the length for the combination of the gas beams must be differently selected.
In accordance with the method according to the invention, it is advantageous if the auxiliary gas beam is pulsed. In this way, the best possible high vacuum conditions can be maintained and the sample molecules are not detrimentally affected by collisions among themselves or with auxiliary gas molecules.
It is advantageous if, after a certain distance of combined flow, the gas beam is narrowed in its cross-section. In this way, a rapid spatial expansion of the sample gas in the ionization chamber is avoided andxe2x80x94as explained abovexe2x80x94a larger part of the sample is ionized which results in an increased sensitivity. If the sample gas beam is combined with the auxiliary gas beam along the center axis of the auxiliary gas beam, the narrowing cross-section improves the focussing (transverse to the flow direction) of the sample gas along the center axis of the auxiliary gas beam (higher density of sample molecules). With a focussing of the laser beam on the center axis of the auxiliary gas beam, a substantial sensitivity increase is achieved by a combination of an increased laser power density at the ionization location and an increased sample gas density in the ionization volume.
It is particularly advantageous if the sample gas beam is embedded in a pulsed carrier gas beam since, in this case, the sample gas beam is compressed also in the flow direction of the gas beam. When this compressed sample gas pulse enters the ionization volume, a larger part of the sample gas molecules is ionized with each laser pulse, which results in a more effective utilization of the introduced sample gas amount and consequently in an increase of the sensitivity.
Preferably, the pulses of the carrier gas beam and of the auxiliary gas beam are correlated such that an advantageous position of the compressed sample gas pulse in the carrier gas pulse is obtained. In this way, an optimal combination of the compression of the sample by the carrier gas pulse in the gas beam flow direction and the compression of the sample by the auxiliary gas flow in a direction transverse to the flow direction can be achieved. As a result, the sample volume can be approximated the ionization volume in the laser beam. With an additional time-synchronization of the gas pulses with the laser pulse, the sample pulse can be controlled so as to be in the ionization chamber exactly at the point in time when the laser pulse illuminates the ionization volume. In this way, no sample molecules are wasted between the laser pulses (if sample molecules are not ionized, they are lost for the detection procedure). This results in a maximal utilization of the sample volume introduced and, consequently, in a substantial increase of the sensitivity when compared with conventional pulsed supersonic molecular beam inlet techniques.
It is advantageous if, after a reduction of the flow cross-section, the gas beam is expanded into a vacuum. Because of the reduction in the flow cross-section, even relatively small volume flows and gas reservoir pressures are sufficient to form a supersonic molecular beam. In this supersonic molecular gas beam, an adiabatic expansion of the gas beam occurs whereby the gas molecules are cooled which, as explained earlier, increases the optical selectivity of the process.
A narrowing of the sample gas tube ahead of its exit into the auxiliary gas guide tube is on one hand advantageous for the compression of the sample gas since, upon introduction of the pulsed carrier gas, the sample is backed up at its discharge opening into the auxiliary gas guide tube and is not pushed into the auxiliary gas beam. The backing up however results in a slower emptying of the sample gas tube whereby a time-wise longer sample gas pulse is generated. On the other hand, the narrowed guide tube provides for a focussing of the sample gas beam on the center axis of the auxiliary gas beam.
The narrowing of the sample gas beam and/or the combined gas beam can be achieved in different ways. For the combination of the beams, constrictions in the form of Laval or Venturi nozzles were found to be advantageous. It is possible to use different nozzle forms for the orifice of the sample gas guide tube into the auxiliary gas guide tube and the orifice of the auxiliary gas guide tube into the vacuum.
It is also advantageous to use a nozzle as the orifice for the auxiliary gas guide tube to the vacuum, which consists of an electrically non-conductive material.
In a preferred embodiment of the method according to the invention inert materials such as quartz glass are used on the surfaces of all gas guide tubes with which sample gas comes into contact. In this way, catalytic processes, which could lead to a change of the sample composition, can be avoided.
It is further advantageous for the method according to the invention if the whole sample gas guide structure up to the outlet orifice into the vacuum chamber can be heated so that memory effects caused by condensation of sample gas components on the guide tube walls and supply line walls are prevented.
The object according to the invention is further solved by an arrangement for generating a directed gas beam, wherein a guided sample gas beam and a directed guided auxiliary gas beam, which extends in the same direction as the sample gas beam but is separated therefrom, is generated and the sample gas beam is subsequently conducted with the auxiliary gas beam together over a certain distance.
The device according to the invention has the advantage that the sample gas beam is so contained in the auxiliary gas beam that it is guided along the center axis of the auxiliary gas beam whereby a rapid spatial expansion of the sample gas beam during expansion into a vacuum is substantially prevented.
An embodiment of the invention will be described below on the basis of the accompanying drawings.